Press molding apparatus and press molding method

ABSTRACT

A press molding apparatus includes upper and lower mother molds  102  and  104 . Each of the mother molds  102  and  104  has four molding surfaces arranged in a single line and satisfies the relationship given by L×α×ΔT/t&lt;0.0008, where L represents the length, t, the thickness, α, the thermal expansion coefficient, and ΔT, the temperature difference between both ends in the thickness direction during induction heating. The press molding apparatus may include a pressing mold set including upper mother molds  102   a  and  102   b  attached to a common fixed shaft  118  through upper supporting shafts  110   a  and  110   b  and lower mother molds  104   a  and  104   b  driven by a common drive shaft  120  through lower supporting shafts  112   a  and  112   b . The upper mother molds  102   a  and  102   b  and the lower mother molds  104   a  and  104   b  are collectively heated by induction heating coils  122  and  124 , respectively.

BACKGROUND OF THE INVENTION

This invention relates to a press molding apparatus and a press moldingmethod for use in precision pressing in which an optical element isobtained by press molding a material, such as a preformed glass materialin a heated and softened state and, in particular, to precision pressingin which no polishing is required after molding.

Recently, in a field of production .of an optical element such as anoptical lens, it is desired to obtain a high-accuracy lens shape withoutcarrying out surface polishing. To this end, proposal is made of amethod comprising the steps of preparing a pre-shaped glass material(preform), heating and softening the preform, and pressing the preformby a high-accuracy pressing surface (Japanese Unexamined PatentPublication JP 2001-10829 A). In particular, use is recently made of amother mold having an elongated shape with a plurality of moldingsurfaces arranged in a single line so as to simultaneously press form aplurality of preforms (Japanese Unexamined Patent Publication JP11-29333).

In the meanwhile, upon producing the optical element by precisionpressing, accuracy and productivity are important aspects.

In this sense, anisothermal pressing (Japanese Unexamined PatentPublication JP 8-133756 A) has contributed to epoch-making progress.Specifically, by shortening a heating cycle of the mother mold ascompared with existing isothermal pressing, the cycle time required toform the glass optical element can be shortened to the order of severaltens of seconds. In addition, surface accuracy and profile accuracy canbe kept superior.

Taking the production efficiency into account, attention is directed toa method of obtaining a plurality of optical elements in one heatingcycle, i.e., a multi-product batch process. As far as the heating cycleis essential and requires a predetermined time period, the productivitycan be improved if a plurality of optical elements are simultaneouslyproduced in the heating cycle.

In the meanwhile, one of design options for the mother mold capable ofsimultaneously producing a plurality of optical elements is to dispose aplurality of molding surfaces in a single-line arrangement (JP 11-29333A mentioned above). Such single-line arrangement is advantageous in thefollowing respects. That is, the structure of the mother mold is simple.In particular, consideration will be made of supply of the glassmaterials to the mother mold. In the state where the glass materials arearranged in a single line, a supplying member is split by a straightline into two parts to drop the glass materials through a gap betweenthe two parts. With such a simple mechanism, the glass materials aresimultaneously supplied onto the mother mold (i.e., to the respectivemolding surfaces).

In order to drop the glass material in a heated and softened state, theglass material in the softened state is floated on a floating saucer bythe use of a gas and then dropped and supplied to the mother mold. Thistechnique is advantageous in that the glass material is stably suppliedwithout damaging the surface of the glass material. For example, byarranging a plurality of floating saucers in a single line and splittingeach floating saucer into two parts, the glass materials aresimultaneously dropped through gaps between the two parts onto themolding surfaces arranged in a single line. In this case, the apparatusis relatively simple in structure. Thereafter, press molding can beimmediately performed before the temperature of the glass material ischanged from the preheat temperature. The above-mentioned technique isvery advantageous in that the productivity is high and a plurality ofoptical elements can be stably and accurately produced under a thermallyuniform condition.

As described above, the linear arrangement of the molding surfaces onthe mother mold is advantageous. However, such design of the mother moldis disadvantageous in the following respects.

If the mother mold having an elongated shape is provided with aplurality of molding surfaces arranged in a single line and if themother mold is warped or deformed due to temperature difference in itsthickness direction, upper and lower molding surfaces are inclined. Theeffect of such inclination is greater towards opposite ends of themother mold in its longitudinal direction. This results in occurrence oftilt in molded products, such as optical lenses, and thickness deviationof the formed products. The tilt and the thickness deviation are greaterin those products formed by the molding surfaces nearer to the oppositeends of the mother mold. Recently, the problem of warp of the mothermold becomes more and more serious. This is because, following therecent demand for reduction in cycle time of the press molding process,rapid heating and rapid cooling are carried out. In addition, followingthe recent demand for simultaneous production of a greater number ofproducts, the mother mold is further elongated.

One of the main factors causing the warp is a temperature gradientproduced in the mother mold, in particular, a temperature gradient inthe vertical direction. In case where the molding surfaces are arrangedin series in a single line, the tilt and the thickness deviation becomemore and more serious under a greater influence of warp of the mothermold as a greater number of molding surfaces are arranged on the mothermold and as the molding surfaces are arranged nearer to the oppositeends of the mother mold.

For example, FIG. 1 shows a basic structure of a typical press moldingapparatus of the type mentioned above. The press molding apparatusillustrated in FIG. 1 has a pressing mold comprising an upper mothermold 502 and a lower mother mold 504. Each of the upper and the lowermother molds 502 and 504 has an elongated shape extending in atransversal or horizontal direction in the figure. The upper and thelower mother molds 502 and 504 are supported by upper and lowersupporting members 506 and 508, respectively. The upper supportingmember 506 is attached to a fixed shaft 510 while the lower supportingmember 508 is attached to a drive shaft 512 of a motor mechanism or thelike. The upper and the lower mother molds 502 and 504 have a pluralityof molding portions 514 and 516 formed on confronting surfaces thereof,respectively, to provide preforms with a lens shape. To a positionbetween the upper and the lower mother molds 502 and 504, the preformseach of which is preliminarily formed into a desired provisional shapeare transferred after heated by a heating unit (not shown) to apredetermined temperature, for example, to a temperature correspondingto a viscosity between 10^(5.6) and 10⁹ poises. The upper and the lowermother molds 502 and 504 are surrounded by induction heating coils 518and 520 for heating the upper and the lower mother molds 502 and 504,respectively. The upper and the lower mother molds 502 and 504, whichare preliminarily heated, clamp and press the preforms in a softenedstate to thereby form high-accuracy processed surfaces on the preforms.

Herein, temperature distribution is caused in the upper and the lowermother molds 502 and 504 in their thickness directions. This is becausethe heat of the upper and the lower mother molds 502 and 504 isdissipated through the supporting members 506 and 508, respectively.Such temperature difference may possibly results in occurrence of thewarp in the upper and the lower mother molds 502 and 504 asschematically depicted by dot-dot-dash lines in FIG. 1. If press moldingis performed in the state where the upper and the lower molds arewarped, defective molding may be caused and the upper or the lowermother mold 502 or 504 may be damaged. If the degree of parallelism isdecreased due to the warp, upper and lower surfaces of press formedproducts will be inclined. If the required product specification isstrict, a desired performance may not be achieved.

In particular, it is recently proposed to further increase the lengthsof the upper and the lower mother molds 502 and 504 so as tosimultaneously form a greater number of (for example, six) preforms forthe purpose of improving the production efficiency and to form lenseshaving greater diameters. Under the circumstances, it is an urgentdemand to suppress the warp.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a press moldingapparatus and a press molding method, which are capable of improving thedimensional accuracy of an optical element by suppressing the warp of amother mold.

It is another object of this invention to provide an apparatus and amethod for producing a glass optical element, which are capable ofobtaining a high-accuracy optical element with high productivity andwithout product variation even if grinding or polishing is not carriedout after molding.

According to this invention, there is provided a press molding apparatusfor simultaneously press molding a plurality of materials into aplurality of optical elements, comprising at least one upper mother moldand at least one lower mother mold, each having a shape extending in adirection; a plurality of upper molding surfaces and a plurality oflower molding surfaces aligned on the at least one upper mother mold andthe at least one lower mother mold, respectively, in the direction; aheater for heating the upper and/or the lower mother molds; upper andlower supporting members each for supporting the at least one uppermother mold and the at least one lower mother molds so that the uppermolding surfaces and the lower molding surfaces are faced to each other;and a drive shaft coupled to the upper supporting member or the lowersupporting member so that the upper mother mold or the lower mother moldmove towards and away from each other for press molding; wherein each ofthe upper and the lower mother molds satisfies the relationship givenby:L×α×ΔT/t<0.0008,  (1)where L represents the length (mm) of the mother mold in the direction,t, the thickness (mm) of the mother mold, α, the thermal expansioncoefficient (/° C.) of the mother mold, and ΔT, the temperaturedifference (° C.) produced between both ends in the thickness directionof the mother mold during the press molding.

With the above-mentioned structure, the mother mold having an elongatedshape can be prevented from being warped. It is consequently possible toimprove the dimensional accuracy of the optical elements formed by themolding surfaces, to suppress the tilt, and to improve the accuracy ofthe lens thickness.

Preferably, the press molding apparatus further comprises a plurality ofthe upper mother molds and a plurality of the lower mother molds. Theplurality of the upper and the lower molding surfaces are aligned on theplurality of the upper mother molds and the plurality of the lowermother molds in the direction.

Preferably, the heater comprises a single induction heating coilsurrounding the at least one upper mother mold or the at least one lowermother mold.

Preferably, in case where a plurality of the mother dies are disposed sothat the molding surfaces are arranged in a single line, each of themother dies has a supporting shaft.

Preferably, the press molding apparatus further comprises a pressingmold set comprising a plurality of the upper mother molds and aplurality of the lower mother molds disposed so that the upper moldingsurfaces and the lower molding surfaces are aligned in the direction;and a plurality of support shafts each supporting each of the uppermother molds and the lower mother molds of the pressing mold set. Theupper mother mold and/or the lower mother molds are moved by the driveshaft through the supporting shafts to move towards and away from eachother.

As described above, the pressing mold set preferably comprising aplurality of upper and the lower mother molds is collectively heated bythe heater and the upper and the lower mother molds are supported by theindividual supporting shafts, respectively. With this structure, even incase where a plurality of materials are simultaneously press molded, theoptical elements excellent in dimensional accuracy can be obtained. Thisis because the distances between the molding surfaces and the supportingshafts can be kept substantially smaller, when compared with the singlemother mold having many molding surfaces in a line and a singlesupporting shaft, and the temperature distributions in the upper and thelower mother molds can be kept uniform, thereby keeping the uniformpress conditions at the respective molding surfaces. Even in case wherethe optical elements are simultaneously obtained by press molding by theuse of the upper and the lower mother molds having a plurality ofmolding surfaces adapted to form the optical elements having a diameterof 10 mm or more, the optical elements excellent in dimensional accuracycan be obtained. Furthermore, since the upper and the lower mother moldsare supported by the individual supporting shafts, the press conditionsby the upper and the lower mother molds can be kept substantiallyuniform. In addition, since the pressing mold set comprises a pluralityof the upper and the lower mother molds, each individual mother mold canbe reduced in length. Therefore, even when a plurality of objects aresimultaneously obtained by press molding or even when a plurality ofoptical elements having a medium aperture size or more aresimultaneously obtained by press molding, it is possible to suppress thewarp of the upper and the lower mother molds due to the heat.

Preferably, each of the upper mother molds and each of the lower mothermolds of the pressing mold set have a plurality of the upper moldingsurfaces and the lower molding surfaces, respectively.

Preferably, the press molding apparatus further comprises an inductionheating coil surrounding the pressing mold set to collectively heat theupper and the lower mother molds by induction heating. The upper and thelower mother molds have rounded corners on sides adjacent to each other.

Preferably, the drive shaft has a center axis substantially coincidentwith the center of the pressing mold set in its longitudinal direction.

According to this invention, there is also provided a press moldingmethod for obtaining a plurality of optical elements by simultaneouslypress molding a plurality of materials, comprising the steps ofpreparing a molding apparatus comprising at least one upper mother moldand at least one lower mother mold, each having a shape extending in adirection, a plurality of upper molding surfaces and a plurality oflower molding surfaces being aligned on the at least one upper mothermold and the at least one lower mother mold, respectively, in thedirection, and the upper molding surfaces and the lower molding surfacesbeing faced to each other; heating the upper and the lower mother molds;and press molding the materials with the upper and the lower moldingsurfaces by driving the upper or the lower mother mold, wherein each ofthe upper and the lower mother molds satisfies the relationship givenby:L×α×ΔT/t<0.0008,  (1)where L represents the length (mm) of the mother mold in the direction,t, the thickness (mm) of the mother mold, α, the thermal expansioncoefficient (/° C.) of the mother mold, and ΔT, the temperaturedifference (° C.) produced between both ends in the thickness directionof the mother mold during the press molding.

Preferably, the press molding apparatus comprises a plurality of theupper mother molds and a plurality of the lower mother molds Theplurality of the upper molding surfaces and the lower molding surfacesare aligned on the plurality of the upper mother molds and a pluralityof the lower mother molds, respectively, in the direction.

Preferably, the heating is carried out by a single induction heatingcoil surrounding at least one of the upper or the lower mother molds.

Preferably, the press molding apparatus comprises a pressing mold setcomprising a plurality of the upper mother molds and a plurality of thelower mother molds disposed so that the upper molding surfaces and thelower molding surfaces are aligned in the direction. The press moldingis carried out by press molding the materials with the upper and thelower molding surfaces by driving the upper mother molds or the lowermother molds, each of the upper mother molds and the lower mother moldsbeing supported by a support shaft.

Preferably, the materials are heated to a temperature higher than thatof the upper and the lower mother molds and softened before thematerials are supplied to the molding surfaces.

Preferably, the materials are glass materials which are heated to atemperature corresponding to the viscosity not higher than 10⁹ poisesbefore the materials are supplied to the molding surfaces.

Other objects of this invention will become clear as the descriptionproceeds.

BRIEF DESCEIPTION OF THE DRAWINGS

FIG. 1 shows an existing press molding apparatus;

FIG. 2 shows a press molding apparatus according to a first embodimentof this invention;

FIG. 3 is a plan view of a lower mother mold in FIG. 2;

FIGS. 4A and 4B are views for describing the effect of this invention;

FIG. 5 shows a press molding apparatus as a modification of the firstembodiment;

FIG. 6 shows a press molding apparatus according to a second embodimentof this invention;

FIG. 7 shows a press molding apparatus according to a third embodimentof this invention;

FIG. 8 shows lower mother molds in FIG. 7;

FIG. 9 is a graph showing the result of measurement of tilt;

FIG. 10 shows a press molding apparatus according to a fourth embodimentof this invention; and

FIG. 11 shows a press molding apparatus according to a fifth embodimentof this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, preferred embodiments of this invention will be described in detailwith reference to the drawing.

At first referring to FIG. 2, a press molding apparatus 100 according toa first embodiment of this invention will be described. The pressmolding apparatus 100 is used to produce an optical lens having apredetermined shape by press molding a preform prepared by preliminarilyforming a glass material into a flat spherical shape.

The press molding apparatus 100 comprises an upper mother mold 102 and alower mother mold 104 arranged at upper and lower positions and faced toeach other. Each of the upper and the lower mother molds 102 and 104 ismade of a tungsten alloy and has an elongated shape extending in thetransversal or horizontal direction in the figure.

The upper mother mold 102 has four upper mold members 112 arranged in asingle line. Similarly, the lower mother mold 104 has four lower moldmembers 114 arranged in a single line and faced to the upper moldmembers 112 of the upper mother mold 102, respectively. The upper moldmembers 112 have lower surfaces as molding surfaces for pressing thepreforms while the lower mold members 114 have upper surfaces as moldingsurfaces for pressing the preforms. The lower mother mold 104 isattached to a lower supporting member 108 driven in the verticaldirection. The upper mother mold 102 is attached to an upper supportingmember 106 as a fixed member. The upper and the lower mother molds 102and 104 are surrounded by an induction heating coil 110 forhigh-frequency induction heating. The induction heating coil 110 iswound in a generally elliptical shape substantially along the outercontour or periphery of the upper and the lower mother molds 102 and104.

Each of the upper mold members 112 is provided with a sleeve 116 formedat its outer periphery. The sleeve 116 is fitted to the lower moldmember 114 with a small clearance to be slidable therealong and servesto prevent axial offset between upper and lower surfaces of an opticallens. The upper mother mold 102 is provided with guide pins 118protruding therefrom while the lower mother mold 104 is provided withguide holes 120 to be engaged with the guide pins 118. Each of the uppermold members 112, the lower mold members 114, and the sleeves 116 ismade of sintered silicon carbide with silicon carbide deposited thereonby CVD.

Referring to FIG. 3, the lower mother mold 104 has a generallyelliptical shape with longitudinal opposite ends rounded. The fourmolding surfaces A, B, C, and D (the upper surfaces of the lower moldmembers 114) are arranged at equal intervals in a single line along thelongitudinal direction of the lower mother mold 104. Although not shownin the figure, the upper mother mold 102 is similar in shape in planview to the lower mother mold 104.

In this embodiment, the upper mother mold 102 satisfies the relationshipgiven by:L×α×ΔT/t<0.0008,  (1)where L represents the length (mm), t, the thickness (mm), α, thethermal expansion coefficient (/° C.), and ΔT, the temperaturedifference (° C.) produced between both ends in the thickness directionof the upper mother mold 102 during press molding.

Likewise, the lower mother mold 104 satisfies the above-mentionedrelationship given by the formula (1) with respect to the length L, thethickness t, the thermal expansion coefficient α, and the temperaturedifference ΔT produced between both ends in the thickness direction ofthe lower mother mold 104 during press molding.

When each of the upper and the lower mother molds 102 and 104 satisfiesthe above-mentioned relationship given by the formula (1), the warp ofthe upper and the lower mother molds 102 and 104 during press moldingcan be suppressed.

Referring to FIGS. 4A and 4B, description will be made of the effectachieved by suppressing the warp of the upper and the lower mother molds102 and 104. As illustrated in FIG. 4A, each of upper and lower mothermolds 202 and 204 are heavily warped. As the warp is increased, moldingsurfaces (upper surfaces of lower mold members 114 in the illustratedexample) located at longitudinal opposite ends of each mother mold havegreater inclination angles θ. Furthermore, the distance between theupper and the lower molding surfaces is slightly increased towards thelongitudinal opposite ends. On the other hand, referring to FIG. 4B, theinclination angle θ′ of the molding surface is minimized in thisembodiment by suppressing the warp of the upper and the lower mothermolds 102 and 104. It is therefore possible to improve the dimensionalaccuracy of optical lenses obtained by press molding. Thus, in casewhere the inclination angle θ is excessively large as illustrated inFIG. 4A as a result of a single-line arrangement of the molding surfacesof the mother mold, the length L of the mother mold is reduced so thatthe inclination angle θ′ becomes small as illustrated in FIG. 4B. Inorder to simultaneously obtain a large number of optical elements bypress molding with the mother mold reduced in length, a plurality ofmother molds are used and disposed so that the molding surfaces arearranged in a single line as illustrated in FIG. 5.

Referring to FIG. 5, description will be made of a press moldingapparatus as a modification of the first embodiment. The press moldingapparatus comprises two sets of the upper and the lower mother molds 102and 104 illustrated in FIG. 2. FIG. 5 shows the arrangement of the lowermother molds 104 in plan view. In the press molding apparatus, the twolower mother molds 104 are disposed in the manner such that the lowermold members 114 are arranged in a single line. Although not shown inthe figure, the two upper mother molds 102 are disposed in the mannersimilar to the lower mother molds 104 (i.e., in the manner such that theupper mold members 112 are arranged in a single line). A heating coil400 surrounds the two upper mother molds 102 (not illustrated in FIG. 5)and the two lower mother molds 104 to simultaneously heat the upper andthe lower mother molds 102 and 104 by high-frequency induction heating.In this modification, it is possible to press mold a large number ofpreforms and to suppress the warp of each of the upper and the lowermother molds 102 and 104 by satisfying the above-mentioned formula (1).It is noted here that three or more sets of the upper and the lowermother molds 102 and 104 may be provided. By the use of a plurality ofthe upper and the lower mother molds, the warp of the upper and thelower mother molds is not increased even if the number of the moldingsurfaces is increased. As a consequence, it is possible tosimultaneously obtain a large number of optical elements excellent indimensional accuracy and thickness accuracy.

Referring to FIG. 6, description will be made of a press moldingapparatus 300 according to a second embodiment of this invention. In thepress molding apparatus 300, a gap is formed between the upper mothermold 102 and the upper supporting member 106 and a plurality of spacers302 are inserted into the gap. Likewise, a gap is formed between thelower mother mold 104 and the lower supporting member 108 and aplurality of spacers 304 are inserted into the gap. The presence of thegaps contributes to suppress dissipation of the heat of the upper andthe lower mother molds 102 and 104 through the upper and the lowersupporting members 106 and 108. With this structure, ΔT in the formula(1) is suppressed. Except that the gaps are formed as mentioned above,the press molding apparatus 300 is similar in structure to the pressmolding apparatus 100 according to the first embodiment. Thus, in thisembodiment also, each of the upper and the lower mother molds 102 and104 satisfies the above-mentioned formula (1). In order to suppress ΔT,the supporting shaft may be made of a heat resistant material or thesupporting shaft may be reduced in sectional area.

The above-mentioned modification (FIG. 5) may be applied to the pressmolding apparatus 300 according to the second embodiment. Furthermore,the mother mold may be made of a material having a relatively smallthermal expansion coefficient α. The thickness t of the mother mold maybe appropriately controlled. In any event, by satisfying the formula(1), it is possible to stably and uniformly produce optical elementshaving sufficient accuracy.

Referring to FIGS. 7 and 8, description will be made of a press moldingapparatus according to a third embodiment of this invention. Forexample, the press molding apparatus is used to produce amedium-aperture lens having a diameter of 17 mm by the use of a preformas a material to be press molded. The preform is prepared bypreliminarily forming a glass material into a flat spherical shape. Asillustrated in FIG. 7, the press molding apparatus comprises a pressingmold set including an upper mold 102 and a lower mold 104. Each of theupper and the lower molds 102 and 104 has an elongated shape extendingin the transversal or horizontal direction in the figure and is made of,for example, a tungsten alloy. The upper and the lower molds 102 and 104are surrounded by induction heating coils 122 and 124, respectively. Theinduction heating coils 122 and 124 serve to heat the upper and thelower molds 102 and 104 by high-frequency induction heating,respectively.

The upper mold 102 comprises a pair of left and right upper mother molds102 a and 102 b. The lower mold 104 comprises a pair of left and rightmother molds 104 a and 104 b. The upper mother molds 102 a and 102 b arefaced to the lower mother molds 104 a and 104 b in the verticaldirection, respectively. The upper mother molds 102 a and 102 b haveupper surfaces fixed to a pair of supporting plates 106 a and 106 b,respectively. The supporting plates 106 a and 106 b have upper surfacesattached to upper supporting shafts 110 a and 110 b, respectively. Theupper supporting shafts 110 a and 110 b are attached to a fixed shaft118 through a common base 114. On the other hand, the lower mother molds104 a and 104 b have lower surfaces fixed to a pair of supporting plates108 a and 108 b, respectively. The supporting plates 108 a and 108 bhave lower surfaces attached to lower supporting shafts 112 a and 112 b,respectively. The lower supporting shafts 112 a and 112 b are attachedto a drive shaft 120 through a common base 116. The drive shaft 120 isdriven by a driving mechanism having an AC servo motor to linearly movein the vertical direction. The drive shaft 120 has a center axis 120 acoincident with the center of each of the upper and the lower molds 102and 104 in the longitudinal direction. When the drive shaft 120 moves inthe vertical direction, the upper and the lower molds 102 and 104 areopened and closed.

Referring to FIG. 8, the lower mold 104 (lower mother molds 104 a, 104b) and the induction heating coil 124 therearound are shown in plan viewas seen from the above. The upper mold 102 (upper mother molds 102 a,102 b) is similar in shape in plan view to the lower mold 104 and is notillustrated in the figure. The lower mother molds 104 a and 104 b aresymmetrical in shape with respect to a center position O in thelongitudinal direction of the lower mold 104. The lower mother mold 104a has a pair of long sides 202 extending in the longitudinal direction,an inner short side 204 perpendicular to the long sides 202 (and nearestto the center position O), and an outer short side 206 faced to theinner short side 204 (and farthest from the center position O). Theouter short side 206 defines an arc of a half circle having a radiusequal to a half of the distance between the two long sides 202 (i.e.,the width of the lower mother mold 104 a). The other lower mother mold104 b is symmetrical in shape with the lower mother mold 104 a withrespect to the center position O. Preferably, a gap of 0.5-3 mm isformed between the inner short sides 204 of the lower mother molds 104 aand 104 b.

The induction heating coil 124 is wound around both of the lower mothermolds 104 a and 104 b in a shape corresponding to an outer periphery ofthe lower mother molds 104 a and 104 b. The induction heating coil 122around the upper mother molds 102 a and 102 b is similar in shape inplan view to the induction heating coil 124.

The lower mother mold 104 a has an upper surface provided with threemolding portions 200 for imparting a glass product shape to preforms.Likewise, the lower mother mold 104 b has an upper surface provided withthree molding portions 200. These six molding portions 200 are arrangedin a single line on a center line M defining the center of the lowermother molds 104 a and 104 b in the widthwise direction. The six moldingportions 200 have preform pressing surfaces as molding surfaces A, B, C,D, E, and F, respectively.

Each of the lower mother molds 104 a and 104 b has a shape with roundedcorners on an adjacent side at which the lower mother molds 104 a and104 b are adjacent to each other. This is because an angled portion isexcessively elevated in temperature under high-frequency inductionheating. By rounding these corners, temperature distribution in each ofthe lower mother molds 104 a and 104 b is kept uniform. Specifically,each of the corners between the inner short side 204 and the long sides202 in each of the lower mother molds 104 a and 104 b has a curve R.Instead of the curve R, the corners may be chamfered. Alternatively, theinner short sides 204 on the adjacent sides of the lower mother molds104 a and 104 b may have a curved shape.

Description will be made of the arrangement of the molding surfaces A toF in each of the lower mother molds 104 a and 104 b. The interval(arrangement pitch) d1 between the molding surfaces A and B, theinterval d2 between the molding surfaces B and C, the interval d4between the molding surfaces D and E, and the interval d5 between themolding surfaces E and F are substantially equal to one another. Inaddition, the interval d0 between the molding surface A and the outershort side 206 and the interval d6 between the molding surface F and theouter short side 206 are equal to the above-mentioned interval (d1 etal). On the other hand, the interval d3 between the molding surfaces Cand D adjacent to each other with the center position O interposedtherebetween is greater than the above-mentioned interval (d1 et al).The shortest distance d7 from the molding surface C to the inner shortside 204 is substantially equal to the shortest distance d8 from themolding surface C to the long side 202. Likewise, the shortest distancefrom the molding surface D to the inner short side 204 is substantiallyequal to the shortest distance from the molding surface D to the longside 202. The above-mentioned relationship of d0 to d8 is determined inorder to minimize the temperature difference in the molding surfaces Ato F during high-frequency induction heating.

The position of the lower supporting shaft 112 a (FIG. 7) in thehorizontal plane corresponds to the center position (i.e., the moldingsurface B) of the molding surfaces A to C of the lower mother mold 104 ain its alignment direction. Likewise, the position of the lowersupporting shaft 112 b (FIG. 7) in the horizontal plane corresponds tothe center position (i.e., the molding surface E) of the moldingsurfaces D to F of the lower mother mold 104 b in its alignmentdirection. With this structure, the pressure is uniformly applied to thepreforms on the molding surfaces A to F. Similarly, the positions of theupper supporting shafts 110 a and 110 b in the horizontal planecorrespond to the center positions of the upper mother molds 102 a and102 b, respectively.

Each of the lower mother molds 104 a and 104 b has an inner region nearto the center position O. In the inner region, the temperature is easilyelevated as compared with the remaining region. Therefore, it ispreferable to provide an air cooling unit for locally cooling theabove-mentioned inner region, thereby achieving uniform temperaturedistribution in the lower mother molds 104 a and 104 b. Preferably, asimilar cooling unit is provided for the upper mother molds 102 a and102 b.

Next, description will be made of specific examples of theabove-mentioned embodiments.

At first, as examples 1 to 4, a biconvex lens having a diameter of 6.6mm was produced by the use of the press molding apparatus 100 (FIG. 2)according to the first embodiment. In the examples 1 to 4, the length L,the thickness t, the width W, the thermal expansion coefficient α, thetemperature difference ΔT between both ends in the thickness directionof the mother molds 102 and 104 in the press molding apparatus 100 werevaried as shown in Table 1 (will later be given). In the examples 1-3and the example 4, the mother molds 102 and 104 were made of tungstenalloys different in thermal expansion coefficient from each other.

Use was made of flat spherical glass preforms (each having the weight of54 mg) of barium borosilicate glass (having a transition point of 514°C. and a sagging point of 545° C.). The glass preforms were supplied tofour floating saucers on a support arm (not shown) and floated up by agas to be supported in a floating state. The glass preforms in thisstate were placed in a heating chamber (not shown) together with thesupporting member. The heating chamber was kept at an atmosphere of 700°C. by PYROMAX (PX-DS). The four glass preforms were collectively rapidlyheated in the heating chamber for a predetermined time period inconformity with a molding cycle speed of the optical glass elementsuntil the glass preforms were heated to about 596° C. (corresponding to10⁸ poises). On the other hand, the four molding surfaces were preheatedby heating mother molds to about 550° C. (corresponding to 10^(10.2)poises of the glass preforms) +3° C. Thereafter, the support arm wasplaced at a position directly above the lower mother molds 104. Byquickly opening the support arm, the floating saucers were split tosimultaneously drop and transfer the preforms from the floating saucersto the molding surfaces of the lower mother mold 104, respectively.Then, the support arm was immediately retreated from the positiondirectly above the lower mother mold 104. High-frequency power wasinterrupted. The lower mother mold was moved upward and pressing wascarried out under the pressure of 60 kg/cm². The lenses were cooled downto 470° C. under the pressure applied by the weight of the mold member112 alone. Thereafter, the lower mother mold 104 was moved downward toseparate or part the upper and the lower mother molds 102 and 104 fromeach other. By the use of a suction pad (not shown), the lenses wereremoved. The cycle time of the press molding process depends upon theability of a high-frequency power supply and the size of the mother mold(heat capacity), etc. When the mother mold having the length of 190 mmwas used, the cycle time was equal to 180 seconds. When the mother moldhaving the length of 112 mm was used, the cycle time was 110 seconds.

For the optical lenses obtained by press molding as mentioned above,tilt were measured. The measurement was carried out for those opticallenses formed by the endmost molding surface (the molding surface A or Din FIG. 3) among the four molding surfaces. Specifically, the edgethickness of the optical lens was measured by a micrometer. The tiltwere calculated from the maximum value and the minimum value of the edgethickness and the lens diameter. The results of measurement are shown inTable 1. TABLE 1 Ex- t L W α ΔT tilt ample (mm) (mm) (mm) (10-7/° C.) (°C.) L · α · ΔT/t (min) 1 30 112 37 52 41 0.000796 1.8 2 30 190 40 52 360.001186 3.2 3 25 190 40 52 35 0.001383 4.1 4 30 112 37 64 42 0.0010042.3 5 30 190 40 52 18 0.000593 1.4 6 30 132 48 52 20 0.000458 0.9 7 30112 37 64 30 0.000672 1.8

On the other hand, as examples 5-7, a biconvex lens having a diameter of6.6 mm was formed by pressing barium borosilicate glass using the pressmolding apparatus 300 (FIG. 6) according to the second embodiment. Inthe examples 5 to 7, the length L, the thickness t, the width W, thethermal expansion coefficient α, the temperature difference ΔT betweenboth ends in the thickness direction of the mother molds 102 and 104were varied as shown in Table 1. In the examples 5, 6 and the example 7,the mother molds 102 and 104 were made of tungsten alloys different inthermal expansion coefficient from each other. The press moldingtechnique actually used was similar to that used in the examples 1-4.For the optical lenses obtained by press molding, tilt was measured. Theresults of measurement is shown in Table 1. Based on the results ofmeasurement, L·α·ΔT/t was calculated for each of the examples 1 to 7.FIG. 9 shows the relationship between the values of L·α·ΔT/t and thetilt.

As will be understood from FIG. 9, L·α·ΔT/t is substantiallyproportional to the tilt. As L·α·ΔT/t is smaller, the tilt is smaller.Generally, the allowance of the tilt in the optical lens is within twominutes. From FIG. 9, it is understood that L·α·ΔT/t is not greater than0.0008 in order to suppress the tilt to two minutes or less.

As seen from Table 1, the tilt is smaller in the examples 5 to 7 than inthe examples 1 to 4. Presumably, this is because the presence of thegaps between the mother molds 102 and 104 and the supporting members 106to 108 suppresses the heat conduction from the mother molds to thesupporting members and the temperature difference ΔT (to therebysuppress L·α·ΔT/t).

As described above, in the press molding apparatus in each of theforegoing embodiments, the mother molds 102 and 104 satisfy thecondition of L·α·ΔT/t not greater than 0.0008. Therefore, it is possibleto suppress the warp of the mother molds 102 and 104 during pressmolding and to thereby improve the dimensional accuracy of the opticallenses.

In particular, in the press molding apparatus according to the secondembodiment, the gaps are formed between the mother molds 102 and 104 andthe supporting members 106 and 108, respectively. Therefore, thetemperature difference ΔT between the both ends in the thicknessdirection of the mother molds 102 and 104 is reduced so that the warp isfurther suppressed. As a consequence, the dimensional accuracy of theoptical lenses can further be improved.

The foregoing embodiments may be modified in various other mannerswithin the scope of the appended claims. For example, each of the upperand the lower mother molds 102 and 104 has four molding surfaces in theforegoing embodiments. However, the number of the molding surfaces ineach mother mold may be any desired number. In the foregoingembodiments, the upper mother mold 102 and the upper mold members 112are separate components. However, the upper mother mold 102 and theupper mold members 112 may be implemented by an integral structure.Similarly, the lower mother mold 104 and the lower mold members 114 maybe implemented by an integral structure.

In the foregoing embodiments, the anisothermal press molding was used.However, this invention is applicable in isothermal press molding inwhich the mother mold and the preforms put in the mother mold are heatedtogether. In the foregoing embodiments, use is made of the mother moldholding a plurality of mold members having molding surfaces,respectively. Alternatively, the mother mold itself may have a pluralityof molding surfaces.

Next, description will be made of a method of producing a lens (as aglass optical element) according to the third embodiment. At first, byhigh-frequency induction heating of the induction heating coils 122 and124 of FIG. 7, the upper mother molds 102 a and 102 b and the lowermother molds 104 a and 104 b are heated, respectively. Next, thepreforms preliminarily formed into a flat spherical shape are suppliedto the lower mother molds 104 a and 104 b by the use of a transfer arm(not shown) after the preforms are preheated to a temperature higherthan that of the upper mother molds 102 a and 102 b and the lower mothermolds 104 a and 104 b. In order to supply the preforms to the lowermother molds 104 a and 104 b, the preforms are positioned above themolding surfaces A to F of the lower mother molds 104 a and 104 b by theuse of a positioning member (not shown) and are dropped and suppliedonto the lower mother molds 104 a and 104 b. Thereafter, the drive shaft120 is moved upward to close the upper mother molds 102 a and 102 b andthe lower mother molds 104 a and 104 b through the upper supportingshafts 110 a and 110 b and the lower supporting shafts 112 a and 112 b.As a consequence, the six preforms are pressed between the upper mothermolds 102 a and 102 b and the lower mother molds 104 a and 104 b to formlenses having a desired shape. After completion of pressing of thepreforms, the drive shaft 120 is moved downward to open or separate theupper and the lower molds 102 and 104. Subsequently, by the use of aremoving member (not shown), six glass optical elements left on thelower mold 104 are sucked and removed. Thus, the lenses (glass opticalelements) surface-processed with high precision are obtained.

As described above, in the press molding apparatus according to thethird embodiment, press molding is carried out by the upper mother molds102 a and 102 b and the lower mother molds 104 a and 104 b. Therefore,in case where a large number of (for example, six) preforms are pressed,each of the upper mother molds 102 a and 102 b and the lower mothermolds 104 a and 104 b has a relatively small length. As a consequence,it is possible to suppress the effect of the warp of the upper mothermolds 102 a and 102 b and the lower mother molds 104 a and 104 b due tothe temperature distribution in the mother molds in the thicknessdirection and to prevent defective molding or damage resulting from thewarp. Since the distance between the molding surfaces A to F and thesupporting shafts can be suppressed to be small, temperature variationamong the molding surfaces can be suppressed and the temperaturevariation among the mother molds can be suppressed. Thus, the pressconditions in the respective molding surfaces can be kept uniform.

Furthermore, the upper mother molds 102 a and 102 b are supported by theupper supporting shafts 110 a and 110 b while the lower mother molds 104a and 104 b are supported by the lower supporting shafts 112 a and 112b. Therefore, it is possible to equalize the press conditions (such aspressing pressure) of the upper mother molds 102 a and 102 b and thelower mother molds 104 a and 104 b. The upper supporting shafts 110 aand 110 b are attached to the single common fixed shaft 118 while thelower supporting shafts 112 a and 112 b are attached to the singlecommon drive shaft 120. Therefore, pressing can be accurately performedby the use of the single driving mechanism.

In addition, the molding surfaces A to F are arranged in a single linein each of the upper mother molds 102 a and 102 b. Therefore, efficienttransfer using the transfer arm or the like can be carried out so thatthe productivity is improved.

The upper supporting shafts 110 a and 110 b support the centers of theupper mother molds 102 a and 102 b, respectively. The lower supportingshafts 112 a and 112 b support the centers of the lower mother molds 104a and 104 b, respectively. Therefore, it is possible to apply uniformpressure to the preforms on the molding surfaces A to F.

The gap of 0.5-3 mm is kept between the upper mother molds 102 a and 102b. Therefore, no interference is caused between the upper mother molds102 a and 102 b to thereby perform excellent pressing. Likewise, the gapof 0.5-3 mm is kept between the lower mother molds 104 a and 104 b.Therefore, no interference is caused between the lower mother molds 104a and 104 b to thereby perform excellent pressing.

In addition, the corners on the adjacent sides of the lower mother molds104 a and 104 b are rounded. Therefore, the temperature distribution ofthe lower mother molds 104 a and 104 b can be kept further uniform.Likewise, the corners on the adjacent sides of the upper mother molds102 a and 102 b are rounded. Therefore, the temperature distribution ofthe upper mother molds 102 a and 102 b can be kept further uniform.

Next, description will be made of a press molding apparatus according toa fourth embodiment of this invention. Referring to FIG. 10, the pressmolding apparatus comprises a pressing mold set including a pair ofpressing molds 60 each of which comprises a mother mold 600 having anelongated shape and four pairs of upper and lower mold members 602 and604 supported by the mother mold 600 to be vertically movable. In FIG.10, only one of the two pressing molds 60 is illustrated. In eachpressing mold 60, the upper mold members 602 and the lower mold members604 are respectively arranged in a single line. The two pressing molds60 are placed so that the upper mold members 602 and the lower moldmembers 604 are respectively arranged in a single line (in thetransversal or horizontal direction in FIG. 10). The pressing mold setis surrounded by an induction heating coil (not shown) wound in agenerally elliptical shape substantially along the outer contour of thepressing mold set.

Each of the upper and the lower mold members 602 and 604 is made ofcemented carbide and has a molding surface (i.e., a surface for pressinga preform P) coated with a thin film of a precious metal alloy. Themother mold 600 is made of a tungsten alloy and has a thermal expansioncoefficient slightly greater than that of cemented carbide. The pressingmolds 60 are supported on a tray 606 which is attached to an upper endof a lower supporting shaft 612 driven in the vertical direction. Abovethe lower supporting shaft 612, an upper supporting shaft 610 as a fixedshaft is arranged. By moving the lower supporting shaft 612 upward, theupper mold members 602 are brought into contact with a head (lower endface) of the upper supporting shaft 610. As a consequence, pressing isperformed between the upper and the lower mold members 602 and 604.

The two pressing molds 60 are similar in shape in plan view to the lowermother molds 104 a and 104 b illustrated in FIG. 8, except the number ofthe upper and the lower mold members. In the two pressing molds 60, eachof the corners on the adjacent sides is shaped into a curve R orchamfered.

As a specific example, a biconvex lens having an outer diameter of 15 mmwas produced by the use of the above-mentioned press molding apparatusand a spherical preform P of barium borosilicate glass (having atransition point of 512° C. and a sagging point of 545° C.).Specifically, the spherical preform P was placed between each of theupper mold members 602 and each of the lower mold members 604 of themother mold 600. The mother mold 600 was mounted on the tray 606 andintroduced into a molding chamber (not shown) kept in an inactiveatmosphere. The tray 606 was placed on the lower supporting shaft 612(FIG. 7). Thereafter, the lower supporting shaft 612 was moved upward sothat the pressing molds 60 are located inside the induction heatingcoil. The induction heating coil is supplied with a high-frequencycurrent to induction heat the mother mold 600. At this time, thetemperature of each of lower molding surfaces (upper surfaces of thelower mold members 604) was measured by a mold temperature monitoringthermocouple inserted into each of the lower mold members 604. On theother hand, the temperature of each of upper molding surfaces (lowersurfaces of the upper mold members 602) was measured by a moldtemperature monitoring thermocouple inserted into each of the upper moldmembers 602. As a result, the temperature deviation among the lowermolding surfaces and the upper molding surfaces during the inductionheating was not greater than +10° C. Then, the preform was heated by theinduction heating coil to 596° C. (the temperature corresponding to theglass viscosity of 10⁸ poises). Thereafter, the lower supporting shaft612 was further moved upward to bring the upper surfaces of the uppermold members 602 into contact with the head of the upper supportingshaft 610 so that the preform P in a softened state was pressed.Subsequently, the lenses thus formed were cooled to a temperature nothigher than the glass transition point. Thereafter, the lower supportingshaft 612 was moved downward and the lenses were removed together withthe mother mold. As a result, the lenses thus obtained were excellent indimensional accuracy with less astigmatic aberration and withoutnonuniform extension.

Referring to FIG. 11, description will be made of a press moldingapparatus according to a fifth embodiment of this invention. The pressmolding apparatus according to the fifth embodiment comprises a pressingmold set including a pair of pressing molds 80 each of which comprisesan upper mother mold 802 and a lower mother mold 804 provided with fourupper mold members 812 and four lower mold members 814, respectively. InFIG. 11, only one of the two pressing molds 80 is illustrated. In eachof the pressing molds 80, the upper mold members 812 and the lower moldmembers 814 are respectively disposed in a single line so that themolding surfaces are arranged in a single line. The pressing mold setcomprising the two pressing molds 80 is disposed so that the upper moldmembers 812 and the lower mold members 814 are respectively arranged ina single line (in the transversal or horizontal direction in FIG. 11).The pressing mold set is surrounded by an induction heating coil (notshown) wound in a generally elliptical shape substantially along theouter contour of the pressing mold set. The upper mother mold 802 issupported by an upper supporting shaft 806 as a fixed shaft. The lowermother mold 804 is fixed to a lower supporting shaft 808 driven in thevertical direction.

Each of the upper mold members 812 is provided with a sleeve 816 formedat its outer periphery and fitted to the lower mold member 814 with asmall clearance to be slidable along the lower mold member 814. Thesleeve 816 thus serves to prevent axial offset between upper and lowersurfaces of a lens. The upper mother mold 802 is provided with guidepins 818 while the lower mother mold 804 is provided with guide holes820 to be engaged with the guide pins 818. Each of the upper and thelower mother molds 802 and 804 is made of a tungsten alloy. Each of theupper mold members 812, the lower mold members 814, and the sleeves 816is made of sintered silicon carbide with silicon carbide depositedthereon by CVD.

The two pressing molds 80 are similar in shape in plan view to the lowermother molds 104 a and 104 b illustrated in FIG. 8. In the two pressingmolds 80, each of the corners on the adjacent sides is chamfered orshaped into a curve.

As a specific example, a biconvex lens (one surface being a sphericalsurface, the other surface being an aspherical surface) having an outerdiameter of 10 mm was formed by pressing barium borosilicate glass(having a transition point of 512° C. and a sagging point of 545° C.)using the above-mentioned press molding apparatus. Specifically,preforms of a flat spherical shape prepared by hot molding and having nosurface defect were preheated to 470° C. The preforms, four in number,were supplied onto the lower mold members 814, four in number, of thelower mother mold 804 preheated to about 470° C. Immediately thereafter,the lower mother mold 804 (having the length of 130 mm and the thicknessof 35 mm) was moved upward to be coupled with the upper mother mold 802preheated to 470° C. At this time, the guide pins 818 and the guideholes 820 were engaged with each other and the sleeves 816 were fittedover the lower mold members 814, respectively. By high-frequencyinduction heating by the induction heating coil, the upper and the lowermother molds 802 and 804 were heated so that the preforms were heated to596° C. (i.e., the temperature at which the preform has a viscosity of10⁸ poises). At this time, the temperature deviation among lower moldingsurfaces (upper surfaces of the lower mold members 814) and uppermolding surfaces (lower surfaces of the upper mold members 812) wasmeasured by the use of a mold temperature monitoring thermocouple. As aresult, the temperature deviation was not greater than +10° C.Thereafter, the lower mother mold 804 was moved upward to perform pressmolding at the pressure of 70 kg/cm². At completion of the pressing, thetemperature different in the thickness direction was about 40° C., andL×α×ΔT/t was 0.00077. After completion of the pressing, the lenses thusformed were cooled at a cooling rate of 50° C./min to a temperature nothigher than the glass transition point. At this time, each of the uppermold members 812 followed the shrinkage of the lens and the lens wascooled under the weight of the upper mold member 812 alone. In otherwords, the upper surface of the lens was kept in contact with the uppermold member 812 during cooling. When the temperature was lowered to 490°C., the lower mother mold 804 was moved downward to separate the upperand the lower mother molds 802 and 804 from each other. The lower mothermold 804 was further moved downward to a lower part of a molding chamber(not shown). By the use of a suction pad, four lenses were removed. Thelenses thus removed may thereafter be annealed if desired. In thesemolds (the upper and the lower mother molds 802 and 804, the upper andthe lower mold members 812 and 814), substantially uniform heating andcooling were performed. The lenses thus obtained were high indimensional accuracy and excellent in surface quality. In addition,eccentricity or decenter after centration was excellent.

Next, description will be made of a sixth embodiment of this invention.A press molding apparatus according to the sixth embodiment comprises apressing mold set including a pair of pressing molds which is similar instructure to those of the fifth embodiment except that an upper mothermold 802 and a lower mother mold 804 comprise three upper mold members812 and three lower mold members 814, respectively. Each of the upperand the lower mother molds and the upper and the lower mold members issimilar in structure to that in the fifth embodiment (except the numberof the upper and the lower mold members). Similar parts are designatedby like reference numerals. The two pressing molds are similar in shapein plan view to the lower mother molds 104 a and 104 b illustrated inFIG. 8. In the two mother molds, each of the corners on adjacent sidesis chamfered or shaped into a curve.

As a specific example, a biconvex lens having a diameter of 10 mm wasformed by the use of the above-mentioned press molding apparatus. Atfirst, the upper and the lower mother molds 802 and 804 (having thelength of 100 mm and the thickness of 35 mm) were induction heated byinduction heating coils to obtain mold temperatures shown in Table 2.Three kinds of the mold temperatures were set as shown in Table 2. Thetemperature deviation among lower molding surfaces (upper surfaces ofthe lower mold members 814) and upper molding surfaces (lower surfacesof the upper mold members 812) was measured by the use of a moldtemperature monitoring thermocouple. As a result, the temperaturedeviation was not greater than +10° C. TABLE 2 at the start of pressingreleasing preform temperature mold temperature temperature (viscosity(poise)) (viscosity (poise)) (° C.) 680 (10^(5.8))  549 (10^(10.2)) 485643 (10^(6.8)) 567 (10^(9.2)) 495 615 (10^(7.4)) 590 (10^(8.2)) 505

Then, three preforms were floated on a transfer arm (not shown) by gasstream and transferred. Thereafter, the transfer arm was placed at aposition directly above the three lower mold members 814 and thepreforms were simultaneously dropped and supplied onto the lower moldmembers 814. The preforms were preheated at three different preheattemperatures as shown in Table 2. Thereafter, the transfer arm wasimmediately retreated from the position above the lower mother mold 804.The induction heating coils were deenergized. A lower supporting shaft808 was moved upward and pressing was performed under the pressure of 70kg/cm². At completion of the pressing, the temperature difference in thethickness direction was 39° C. which leads that L×α×ΔT/t of 0.00058.After completion of the pressing, the lenses thus formed were cooleddown to the temperature not higher than the glass transition point.During cooling, the lenses were applied with the weight of the uppermold member 812 alone. Thereafter, the lower mother mold 804 was moveddownward by about 40 mm to separate or part the upper and the lowermother molds 802 and 804 from each other. By the use of a suction pad,the lenses were removed. By the induction heating coil, the upper andthe lower mother molds were immediately recovered to a pressing starttemperature to execute a next molding cycle in the similar manner.

As a result, under any molding condition shown in Table 2, high-qualitylenses were continuously obtained. Thus, it is understood that,according to this embodiment, a large amount of lenses can becontinuously produced with high efficiency.

In the third through the sixth embodiments, the pressing mold setcomprises the upper mold 102 and the lower mold 104 each of whichincludes the two mother molds. Alternatively, the number of the mothermolds may be three or more. Preferably, these mother molds are equal inwidth to one another (in the longitudinal direction of the pressing moldset). In the third through the sixth embodiments, each of the upper andthe lower molds 102 and 104 has the three or four molding surfaces.However, the number of the molding surfaces may be smaller or greater.Alternatively, each mother mold may have only one molding surface.

As described above, according to this invention, the mother moldsatisfies the relationship given by L·α·ΔT/t<0.0008 with respect to thelength L, the thickness t, the thermal expansion coefficient α, and thetemperature difference ΔT produced between the both ends in thethickness direction during press molding. Therefore, it is possible tosuppress the warp of the mother mold, thereby improving the dimensionalaccuracy of the optical element formed by each molding surface.

As described above, according to this invention, the pressing mold setcomprising a plurality of mother molds is collectively heated by theinduction heating coil and the mother molds are supported by theindividual supporting shafts, respectively. With this structure, even incase where a plurality of materials are simultaneously press formed, theoptical elements excellent in dimensional accuracy can be obtained. Thisis because the distances between the molding surfaces and the supportingshafts can be reduced and the temperature distributions in the mothermolds can be kept uniform, thereby keeping the uniform press conditionsat the respective molding surfaces. Even in case where the opticalelements are simultaneously obtained by press molding by the use of themother molds having a plurality of molding surfaces adapted to form theoptical elements having a diameter of 10 mm or more, the opticalelements excellent in dimensional accuracy can be obtained.

Furthermore, since the mother molds are supported by the individualsupporting shafts, the press conditions by the respective mother moldscan be kept substantially uniform. In addition, since the pressing moldset comprises a plurality of mother molds, each individual mother moldcan be reduced in length. Therefore, even when a plurality of materialsare simultaneously pressed or even when a plurality of optical elementshaving a medium aperture size or more are simultaneously obtained bypress molding, it is possible to satisfy the relationship (1) and thusto suppress the warp of the mother molds due to the heat.

1. A press molding apparatus for simultaneously press molding aplurality of molding materials into a plurality of optical elements,comprising: a plurality of upper mother molds and a plurality of a lowermother molds, each of said plurality of upper mother molds and lowermother molds comprising a plurality of upper molding surfaces and aplurality of lower molding surfaces, respectively, a plurality ofsupporting shaft each of which supports each of the upper mother moldsand each of the lower mother molds, at least one heating coil whichsurrounds the plurality of upper mother molds collectively and theplurality of lower mother molds collectively, and a driving shaft whichmoves the upper mother molds or the lower mother molds so that the uppermother molds or the lower mother molds move towards and away from theothers, wherein the plurality of the upper supporting shafts or theplurality of the lower supporting shafts are commonly driven by saiddriving shaft.
 2. The apparatus of claim 1 wherein said upper mothermolds and said lower mother molds are disposed so that upper moldingsurfaces and the lower molding surfaces are aligned in a direction. 3.The apparatus of claim 1 wherein the heating coil comprises an inductionheating coil.
 4. The apparatus of claim 3 wherein and said upper mothermolds and said lower mother molds each comprises round shaped portion ona side adjacent to each other.
 5. The press molding method for obtaininga plurality of optical elements by simultaneously press molding aplurality of materials, comprising: supplying a molding materials into apress molding apparatus, and carrying out a pressing, wherein saidapparatus comprising a plurality of upper mother molds and a pluralityof a lower mother molds, each comprising a plurality of upper moldingsurfaces and a plurality of lower molding surfaces, respectively, aplurality of supporting shaft each of which supports each of the uppermother molds and the lower mother molds, at least one heating coil whichsurrounds the plurality of upper mother molds collectively and theplurality of lower mother molds collectively, and a driving shaft whichmoves the upper mother molds or the lower mother molds so that the uppermother molds or the lower mother molds move towards and away from theothers, whereby the plurality of the upper supporting shafts or theplurality of the lower supporting shafts are commonly driven by saiddriving shaft.
 6. The press molding method of claim 5 wherein themolding materials are heated to a temperature higher than that of theupper and the lower mother molds and softened before the materials aresupplied.
 7. The press molding method according to claim 5 wherein themolding materials are glass materials which are heated to a temperaturecorresponding to the viscosity not higher than 109 poises before thematerials are supplied.